Nissan's presence at next month's Geneva Motor Show may be dominated by its Leaf e-car and ESFlow concept e-sportster, but it will also be unwrapping a petrol engine it claims will deliver the world's lowest carbon output.
Dubbed the 'DIG-S', the engine, which is being designed for the Micra, pumps out 95g of CO2 for every …

COMMENTS

CVT?

CVT

Continuously Variable Transmission.

http://en.wikipedia.org/wiki/Continuously_variable_transmission

They've been on things like Scooters for ages, but they are beginning to be seen on small cars as well, they normally work by some variation of two cones pointing opposite directions with a rubber band connecting the cones with one cone attached to the engine, and the other cone attached to the power shaft, moving where the band connects with the two cones changes the ratio between them and thus the gear ratio, but with out relying on a limited number predefined gear ratios like in a gear box.

Upside, you can keep the engine revs in a much narrower band and you don't need a clutch which makes for much smoother accerlation.

Downside is they are all based on friction between 2 surfaces rather than mechanically interlocking teeth like you get in a gearbox which means without a huge surface area to provide the friction there is a limit to the amount of torque you can push through it.

CVT = Continuously Variable Transmission

RE CVT

Constant Velocity Transmission... roughly, a system of gear structures that rotate at a constant speed but alter shape to give a higher/lower ratio to match the power input and resistance. (from what I remember of the DAF system that consisted of cone like structures). Thus no 'actual' swapping of cogs.

CVT

A CVT...

...is a continuously variable [automatic] transmission. In other words it does not have fixed gear ratios, but instead has (usually) something akin to a belt drive on two variable diameter pulleys. In theory it should be an ideal transmission, always remaining in a good place in the power band of the engine, however, their actual implementation often leaves a lot to be desired. Hence the (relatively) poor CO2 figure. Also they can feel a bit like driving a mobility scooter. Which is probably great for large section of the expected market of a Micra...

@CaptainHook

"They've been on things like Scooters for ages, but they are beginning to be seen on small cars as well"

Beginning? They've been available on cars for at least 30 years that I can think of. I owned one for a while and it's quite a weird experience. The throttle in effect sets the RPM and then the road speed increases to match. So you floor the throttle and the revs go up to say 6Krpm and stay there until the transmission hits the highest ratio it can hold for that engine speed.

Toyota iQ?

Atkinson cycle

They aren't really catching up. Toyota have had an Atkinson cycle petrol engine in the Prius for 10 years now... Miller is just a variant of that though both are different from the standard Otto cycle operation.

Acceleration

Both Atkinson and Miller are better at constant speeds. They do not offer the acceleration characteristics of an Otto cycle engine. In the Toyota Prius the electric motors are the ones used, with their very inventive transmission system, to supplement the petrol engine when acceleration is needed. This site (which isn't mine!) has a nice animation:

Who's copying whom?

The 'Atkinson' engines used in the Prius aren't real Atkinsons, but Otto cycle engines, modified to use a Miller combustion cycle.

James Atkinson gave his name to a combustion/valve-opening cycle he developed in the late 19th century, having observed and talked to the valve-operators, on the four stroke engines used in pumping out mine water in the later Victorian period. These were fueled by stuff like finely-ground coal dust, blown into the combustion chamber using belllows, and had a very slow rotation rates. It had long been known that a perfect rhythm, in hauling on the valve levers, to open and close them, would lead to the best power output from such engines, and a good valve operator (who knew the rhythm) was a highly prized man.

Atkinson's interest was spurred by this, and he went on to study the effect, described the maths involved, in an idealised valve-overlap system, for a four stroke engine back in 1882: the Atkinson cycle. So good was this work, that the effects, first seen in engines with rotations as low as 1 rpm, still hold true for modern four strokes, of 18,000 rpm.

In order to evade certain patents, held over the Otto cycle engine, however, Atkinson used an ingenious tripple crank mechanism to achieve just one turn of the primary drive crank, per four strokes of the engine. It was totally superfluous to the actual effect, but did get around getting sued by the patent holders. This is why the 'Atkinson' engines in modern cars are not true Atkinsons, but Ottos, that use the Atkinson cycle.

Ralph Miller's idea, of 1940, was to supercharge the inlet of an Otto engine, running an Atkinson cycle, overcoming the low power-density of the Atkinson engine. His engines were intended for Marine and static power-generation use, and were heavily patented. This led to the automotive industry largely disregarding this type of engine for road use - just as Rudolph Diesel's successful patenting of the compression-ignition engine, meant that the less efficient carburated Otto engines were used in most car and motorcycle engines for much of the last century.

So there you go: if you like (and many of you will, of course) we can conclude that patents are so evil, they actually lead to global warming (unless your names happen to be 'Lewis Page' or 'Andrew Orlowski', of course - in which case global warming is caused by Stephen Fry).

Yup, modded Otto cycle all right....

.....but they are Atkinson's. The Miller cycle *requires* a supercharger, so it is more accurate to refer to the unsupercharged Prius engine as an Atkinson rather than a Miller unit. All the rest of the Atkinson gubbins relates to the Atkinson pattern engine as a whole, rather than the Atkinson combustion cycle considered in isolation.

Your "why Otto beat Diesel" bit needs work as the Otto engine was patented too (which is why Atkinson was working round the patents). The reason it succeeded over the diesel was not patent related but merely due to the fact that early Fuel Injection systems required by diesels were hideously expensive, monumentally complicated, heavy and seriously unreliable.

Miller's engines were never adopted for two reasons. Firstly, as the Otto patents had expired by that time there was the licensing cost to consider. Secondly and of most importance, was that at the time fuel economy was only an important consideration for endurance racing events, which is what Miller built his engines for with great success.

Um....

Bad explanation

It's a poor explanation of the Miller cycle. Obviously the stroke will always be the same length, however the intake valve stays open further into the compression stroke therefore reducing the amount of energy wasted on compressing the fuel/air mixture. Obviously this will only work with some sort of forced induction, hence the supercharger.

Now the thing is that the supercharger takes a certain amount of energy to drive, but with the whole system being designed correctly it doesn't sap as much energy as the compression stroke would on a normal Otto engine. The power delivery characteristics of a Miller cycle engine are somewhat different to a normal Otto cycle engine, so it's doubtful you will find these engines in performance cars any time soon.

Road tax ... there is none

I call Bullshit

Burn x amount of carbon (ie petrol/whatever) and you generate y amount of CO2. There is no way to reduce CO2 except by reducing fuel consumption (or by incomplete combustion to emit the hydrocarbons without burning them).

Re: I call Bullshit

Two Stroke?

At least twenty years ago I recall reading that Ford and Yamaha were colaborating on a design for a direct injection two stroke engine which would be significantly more efficient than any four stroke engine. I don't know what happened to that project, but now would seem to be a good time to revive it.

It makes sense as there is less wasted energy on a two stroke. There is no valve gear to drive and no "wasted" strokes, you get one power stroke for every complete revolution of each cylinder which is twice as many as you get with a four stroke engine.

The traditional problem with two strokes in terms of emissions has always been that the lubrication system means that you get an awaful lot of oil burned meaning smoke and the unburned HC level is high. That shouldn't be too hard to overcorme with a seperate and sealed lubrication system for the bearings, the only issue then with lubrication would be feed and return to lubricate the piston rings.

Some people who have experienced two stoke motorcyles assume that two strok engines are inefficient since their bikes used a lot of fuel. This was actually a byproduct of the efficiency of the engine, most two stroke bikes were tuned for power rather than economy. You only have to look at MotoGP to see how this worked, when GP500 became MotoGP four strokes were allowed up to 990cc and the four strokes had no restrictions on the likes of weight and noise. This was the only way they could gaurantee a four stroke would outperform a 500cc two stroke. Ten years ago a 500cc two stroke GP engine was getting close to 400bhp per litre! Yes that's going to use an awful lot of fuel, but if you can make that much power with a two stroke engine imagine the economy you could get if you tuned for that.

My old 350LC would outrun any 550cc four stroke and return about 35mpg doing it (not good for a bike) but ridden gently on a long run it had no problems returning over 50mpg which was actually better than the 550. I always presumed therefore that on a whiff of throttle the two stroke beat the four stroke because it was so much lighter and didn't waste any power driving valve gear and the like. I often wondered how much better the economy would have been had the engine been tuned to give a similar output to a 350cc four stroke.